Method of forming retortable plastic container having improved base stability

ABSTRACT

A method of forming a retortable plastic container, the container including a main body and a bottom. The bottom defines a raised inner portion and at least one substantially flat bottom support surface. The substantially flat bottom support surface is curved and positioned near a radially outermost edge on the bottom when viewed in bottom plan. A groove is defined in the substantially flat bottom support surface. In addition, a first side wall portion that extends upwardly from the radially outermost edge of the bottom is shaped as a truncated cone, giving the bottom portion greater dimensional stability under retort conditions. The container bottom exhibits superior dimensional stability with respect to predecessor designs.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/347,261, filed Jan. 10, 2012, which is incorporated herein by reference in its entirety.

BACKGROUND Field of the Disclosed Subject Matter

This disclosed subject matter relates generally to the field of packaging, and more specifically to the field of retortable plastic containers. More specifically, the disclosed subject matter relates to an improved retortable container that is more dimensionally stable during the sterilization process than were conventional predecessor containers, and methods of making the same.

Description of the Related Technology

Certain products require sterilization during the packaging process in order to inhibit the growth of bacteria. Products requiring sterilization include foods such as milk, yogurt and various sauces and prepared foods, as well as certain pharmaceutical products. Thermal processing, sterilization, canning and retorting are all terms referring to the process of taking a food product, already sealed in its container, and heating it to a specific temperature for a specific time. The objective is to kill spoilage organisms and pathogenic bacteria, thus preserving the food and allowing it to be stored unrefrigerated for extended lengths of time.

There are multiple designs for retorting food containers, including batch systems and continuous systems. In a batch system, containers are placed in crates or baskets, which are then loaded into a vessel into which the heating medium is introduced. This method is the oldest and most traditional and also the most versatile in the range of products and container sizes it can handle. In a continuous retort system, a conveyor is used to continuously transport the containers to be sterilized through a heating chamber that contains the heating medium. There are advantages to each method depending on individual processing operations and, just as important, the type of food being processed.

Traditionally, products that require heat sterilization have been packaged in glass containers, which are relatively stable at elevated temperatures and pressures. However, in recent years plastic retortable containers have come into use. Plastic containers tend to be less expensive than glass containers and safer in many respects because they will not shatter when dropped. Unfortunately, plastic containers may lack the column strength that is necessary to avoid deformation of the sidewall of the container when a number of containers or palettes of containers are stacked during transportation or in packaging or retail facilities. While it is possible to increase the strength of a plastic container by increasing the thickness of the sidewall, doing so also increases manufacturing costs by increasing the amount of plastic material that is required. Lightweighting is an important consideration in the design of plastic containers, including plastic cans, because plastic material tends to be relatively expensive.

Many plastic containers also lack the requisite circumferential or hoop strength that is required to avoid excessive deformation when the contents of the container becomes pressurized, such as during a heat sterilization process.

The most common commercial procedure for heat sterilizing canned foods is a retort process in which filled but unsterilized sealed cans are placed in a retort chamber that is injected with steam and held at a predetermined elevated temperature (typically between about 210° F. to about 260° F.) for a predetermined period of time. Conventional plastic containers have been considered unsuitable for packaging applications in which heat sterilization is required, because the heat and pressurization that is inherent to such processes has the tendency to cause irreversible damage and deformation to the sidewall of the plastic can.

The temperatures of the retort process are elevated enough to temporarily increase the internal pressurization of the container. Plastic retortable containers accordingly have been designed to permit limited and reversible controlled flexure of one or more surfaces in order to accommodate the internal volumetric changes that are inherent to the retort sterilization process. U.S. Pat. No. 5,217,737 to Gygax et al. discloses a retortable plastic container that has a flexible bottom portion to accommodate internal volumetric changes. Other retortable containers that have been in commercial use have a champagne style bottom portion that is designed to permit a certain amount of flexure. However, when using a continuous retort process the flexure of retortable plastic containers must be limited so that it will not interfere with the process of conveying the container through the continuous retort system. Typically, such conveyors require at least two dimensionally stable points of contact on the container.

Is also important that the bottom portions of such containers retain their dimensional stability after heat sterilization so that they will not rock when placed on a horizontal support surface such as a table or retail shelf. Containers that are manufactured using an extrusion blow molding process typically have a mold parting line or seam. In order to prevent the mold parting seam from affecting the stability of the container when it is resting on a horizontal surface, recesses or tunnels are commonly integrated into the container bottom design in the area about the parting seam. In addition, differential cooling that occurs during molding process can result in warping of the container bottom that can result in rocking when the container is placed on a horizontal surface. Such rocking can be exacerbated by additional plastic deformation that occurs during the heat sterilization process. In the past, some designs have compensated for this warping or distortion by molding the container bottom to have a slight undulation. However, the presence of the undulation sometimes acted as an initiation locus or migration point for failure of the container during the heat sterilization process.

A need exists for an improved retortable container that exhibits improved dimensional stability and strength during the heat sterilization process without significantly adding to material costs.

SUMMARY OF THE DISCLOSED SUBJECT MATTER

Accordingly, it is an object of the disclosed subject matter to provide a retortable container that exhibits improved dimensional strength and stability during the heat sterilization process without significantly adding to material costs.

In order to achieve the above and other objects of the disclosed subject matter, a retortable plastic container according to a first aspect of the disclosed subject matter includes a main body and a bottom. The bottom defines a raised inner portion and at least one substantially flat bottom support surface. The substantially flat bottom support surface is curved and positioned near a radially outermost edge of the bottom when viewed in bottom plan. A groove is defined in the substantially flat bottom support surface.

These and various other advantages and features of novelty that characterize the disclosed subject matter are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the disclosed subject matter, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there is illustrated and described a preferred embodiment of the disclosed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a retortable plastic container that is constructed according to a preferred embodiment of the disclosed subject matter;

FIG. 2 is a bottom plan view of the retortable plastic container that is depicted in FIG. 1;

FIG. 3 is a cross-sectional view taken along lines 3-3 in FIG. 2; and

FIG. 4 is an enlarged portion of the area 4-4 that is depicted in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings, wherein like reference numerals designate corresponding structure throughout the views, and referring in particular to FIG. 1, a retortable plastic container 10 that is constructed according to a preferred embodiment of the disclosed subject matter includes a main body 12 and a bottom 14 that is unitary with the main body 12. The bottom 14 is constructed and arranged so as to be stable when placed on an underlying horizontal surface such as a tabletop or retail shelf

Referring again to FIG. 1, the retortable plastic container 10 includes an upper rim 16 defining an opening 18. The main body 12 and the bottom 14 are preferably fabricated from a single unitary sidewall 17 having an outer surface 19 and an inner surface 21 that defines an interior space within the container that is accessible through the opening 18. After the retortable plastic container 10 has been filled, a lid 31 is mounted to the rim 16 in order to seal the plastic container 10 prior to the heat sterilization process. Lid 31 is diagrammatically shown in FIG. 3.

The retortable plastic container 10 is preferably fabricated using an extrusion blow molding process from a plastic material that is compatible with the heat sterilization process, most preferably polypropylene. Most preferably, a multilayer material containing polypropylene and additional layers, such as a barrier layer with low oxygen permeability, may be used to form the sidewall 17 of the plastic container 10.

Referring now to FIGS. 2-4, the container bottom 14 is preferably shaped to define a raised inner portion 20 and at least one substantially flat bottom support surface 22 that is curved when viewed in bottom plan, as is shown in FIG. 2. The substantially flat bottom support surface 22 is preferably positioned near a radially outermost edge 23 of the bottom 14 and further preferably has a groove 24 defined therein.

Groove 24 is preferably shaped so as to be substantially concentric with respect to the curvature of the substantially flat bottom support surface 22, as is best shown in FIG. 2. In the preferred embodiment, the container 10 has a mold parting seam 26 as a result of the extrusion blow molding process, and a pair of opposed tunnel recesses 28, 30 are defined in the substantially flat bottom support surface 22 in the area of mold parting seam 26. Otherwise, the substantially flat bottom support surface 22 has a substantially annular shape defined by a substantially constant inner radius and a substantially constant outer radius, and preferably extends continuously around the outer periphery of the container bottom 14.

As FIG. 4 best shows, the substantially flat bottom support surface 22 has a width W_(S), which is defined as the radial distance of the lowermost surface of the support surface 22. The surface preferably resides substantially within a plane that will contact an underlying horizontal surface on which the container 10 is resting. The container 10 has a maximum outer diameter D_(MAX), as is depicted in FIG. 3. Preferably, a ratio W_(S)/D_(MAX) of the width W_(S) to the maximum outer diameter D_(MAX) is substantially within a range of about 0.035 to about 0.2, more preferably substantially within a range of about 0.0425 to about 0.01 and most preferably substantially within a range of about 0.05 to about 0.008.

The groove 24 preferably has a maximum width W_(G), which is best shown in FIG. 4. Preferably, a ratio W_(G)/W_(S) of the maximum width W_(G) of the groove to the width W_(S) of the support surface is substantially within a range of about 0.1 to about 0.9, more preferably substantially within a range of about 0.25 to about 0.75 and most preferably substantially within a range of about 0.40 to about 0.60.

The groove 24 preferably has a curved inner surface 32 that has an average radius of curvature R_(G), as is best shown in FIG. 4. Preferably, a ratio R_(G)/W_(S) of the average radius of curvature R_(G) to the width W_(S) of the support surface is substantially within a range of about 0.05 to about 1.0, more preferably substantially within a range of about 0.25 to about 0.75 and most preferably substantially within a range of about 0.40 to about 0.60.

The groove 24 also preferably has a maximum depth D_(G), also best shown in FIG. 4. Preferably, a ratio D_(G)/W_(S) of the maximum depth of the group 24 to the width W_(S) of the support surface is substantially within a range of about 0.01 to about 0.30, more preferably substantially within a range of as 0.03 to about 0.20 and most preferably substantially within a range of about 0.05 to about 0.15.

As FIG. 4 also best shows, the outermost edge 23 of the of support surface 22 is preferably convex and has an average radius of curvature R_(O). Preferably, a ratio R_(O)/D_(G) of the average radius of curvature R_(O) of the outermost edge 23 to the maximum depth D_(G) of the groove 24 is preferably within a range of about 0.5 to about 10.0, more preferably substantially within a range of about 0.75 to about 5.0 and most preferably substantially within a range of about 1.0 to about 3.0.

As FIG. 4 also shows, the sidewall 17 in the area of the bottom 14 preferably has an average thickness W_(T). Preferably, a ratio of the average thickness W_(T) to the average radius of curvature R_(G) is substantially within a range of about 0.5 to about 2.0, more preferably substantially within a range of about 0.7 to about 1.8 and most preferably substantially within a range of about 0.9 to about 1.6.

Preferably, a ratio W_(G)/W_(T) of the maximum width of the groove 24 to the average thickness of the side wall 17 is substantially within a range of about 0.1 to about 1.0, more preferably substantially within range of about 0.2 to about 0.85 and most preferably substantially the range about 0.3 to about 0.65.

Referring again the FIG. 4, it will be seen that the sidewall 17 above the outermost edge 23 of the support surface 22 has a first sidewall portion 40 and a second sidewall portion 42. The first sidewall portion 40 is preferably substantially straight when viewed in side elevation and extends circumferentially about the outer periphery of the lower end of the container 10 directly above the outermost edge 23 to form a truncated inverted cone shape. As viewed in longitudinal cross-section as shown in FIG. 4, the first sidewall portion defines an angle a with respect to a vertical axis that preferably substantially within a range of about zero degrees to about 15 degrees, more preferably substantially within a range of about zero degrees to about 12 degrees and most preferably substantially within a range of about zero degrees to about 8 degrees.

The first sidewall portion further has a height H₁. Preferably, a ratio H₁/D_(MAX) of the height H₁ of the first sidewall portion to the maximum outer diameter D_(MAX) of the container 10 is substantially within a range of about 0.015 to about 0.040, more preferably substantially within a range of about 0.020 to about 0.035 and most preferably substantially within a range of about 0.025 to about 0.030.

The second sidewall portion 42 is unitary with the first sidewall portion 40 and a connected thereto by a concave fillet having an average radius of curvature R_(B). Preferably, a ratio R_(B)/D_(MAX) of the average radius of curvature R_(B) to the maximum outer diameter D_(MAX) of the container 10 is substantially within a range of about 0.0045 to about 0.065, more preferably substantially within a range of about 0.007 to about 0.04 and most preferably substantially within a range of about 0.0092 to about 0.02.

The presence of the groove 24 and the division of the support surface 22 into inner and outer portions in conjunction with the truncated cone shape of the first sidewall portion 40 has the effect of stiffening the bottom 14 during the heat sterilization process, and also has the tendency to preserve the flatness of the support surface 22 both during the heat sterilization process and during subsequent use of the container 10. The stiffening effect is particularly pronounced when using polypropylene.

It is to be understood, however, that even though numerous characteristics and advantages of the present disclosed subject matter have been set forth in the foregoing description, together with details of the structure and function of the disclosed subject matter, the disclosure is illustrative only, and changes may be made in detail, especially in matters of shape, size and arrangement of parts within the principles of the disclosed subject matter to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. 

What is claimed is:
 1. A method of forming a retortable plastic container, comprising the steps of: disposing a polymeric material in a mold; blow molding the polymeric material in the mold to form a container comprising: a main body, and a bottom having a raised inner portion and at least one support surface defining a reference plane, the support surface having a radially inward portion and a radially outward portion each co-planar with the reference plane, the support surface further having a curved portion near a radially outermost edge of the bottom as viewed in longitudinal cross-section, the bottom having a groove defined in the support surface between the radially inward portion and the radially outward portion; and removing the container from the mold.
 2. The method of forming a retortable plastic container according to claim 1, wherein the polymeric material comprises polypropylene.
 3. The method of forming a retortable plastic container according to claim 1, wherein disposing the polymeric material includes extruding the polymeric material.
 4. The method of forming a retortable plastic container according to claim 4, wherein the mold defines a mold parting line in the container, and the bottom surface extends continuously around a periphery of the bottom, further wherein a pair of opposed tunnel recesses are defined in the support surface proximate the mold parting line.
 5. The method of forming a retortable plastic container according to claim 1, wherein a bottom of the support surface has a width between the raised inner portion and the curved portion, and the retortable plastic container has a maximum outer diameter, and further wherein a ratio of the width to the maximum outer diameter is substantially within a range of about 0.035 to about 0.2.
 6. The method of forming a retortable plastic container according to claim 1, wherein the groove is substantially concentric with the support surface.
 7. The method of forming a retortable plastic container according to claim 1, wherein the groove has a curved inner surface having a maximum width, and a bottom of the support surface has a width between the raised inner portion and the curved portion, and further wherein a ratio of the maximum width of the groove to the width of the bottom of the support surface is substantially within a range of about 0.1 to about 0.9.
 8. The method of forming a retortable plastic container according to claim 1, wherein the groove has a curved inner surface having an average radius of curvature in longitudinal cross-section and a bottom of the support surface has a width between the raised inner portion and the curved portion, wherein a ratio of the average radius of curvature to the width of the bottom of the support surface is substantially within a range of about 0.05 to about 1.0.
 9. The method of forming a retortable plastic container according to claim 1, wherein the groove has a maximum depth in longitudinal cross-section and a bottom of the support surface has a width between the raised inner portion and the curved portion, and wherein a ratio of the maximum depth of the groove to the width of the bottom of the support surface is substantially within a range of about 0.01 to about 0.3.
 10. The method of forming a retortable plastic container according to claim 1, wherein the groove has a maximum depth in longitudinal cross-section and the radially outermost edge of the support surface is convex and has an average radius of curvature, and a ratio of the average radius of curvature of the outermost edge to the maximum depth of the groove is substantially within a range of about 0.5 to about
 10. 11. The method of forming a retortable plastic container according to claim 1, wherein the container further comprises a first side wall portion extending upwardly from the radially outermost edge, and wherein the first side wall portion is substantially straight when viewed in longitudinal cross-section.
 12. The method of forming a retortable plastic container according to claim 11, wherein the first side wall portion defines an angle with respect to a vertical axis extending perpendicular to the reference plane, the angle being substantially within a range of about 0° to about 15°.
 13. The method of forming a retortable plastic container according to claim 11, wherein the first side wall portion is shaped substantially as a truncated cone.
 14. The method of forming a retortable plastic container according to claim 11, further comprising a second side wall portion extending upwardly and outwardly from the first side wall portion.
 15. The method of forming a retortable plastic container according to claim 14, wherein the second side wall portion is convexly curved in longitudinal cross-section
 16. The method of forming a retortable plastic container according to claim 15, wherein the second side wall portion is shaped substantially as a bowl.
 17. The method of forming a retortable plastic container according to claim 15, wherein the second sidewall extends from the first side wall portion to a sidewall groove.
 18. The method of forming a retortable plastic container according to claim 17, wherein the sidewall groove extends about an entire circumference of the main body portion.
 19. The method of forming a retortable plastic container according to claim 17, wherein a third sidewall extends upwardly and radially inwardly from the sidewall groove.
 20. The method of forming a retortable plastic container according to claim 19, wherein the third sidewall is shaped substantially as a truncated cone. 